1. Field of the Invention
This invention relates to electrical devices comprising conductive polymer compositions.
2. Summary of the Prior Art
Conductive polymer compositions comprising a conductive carbon black dispersed in a polymer are well known. Over recent years, there has been particular interest in such compositions which exhibit positive temperature (PTC) characteristics, i.e. which show a very rapid increase in resistivity over a particular temperature range. Reference may be made for example to U.S. Pat. Nos. 2,978,665; 3,243,753; 3,351,882; 3,412,358; 3,413,442; 3,591,526; 3,673,121; 3,793,716; 3,823,217; 3,858,144; 3,861,029; 3,914,363 and 4,017,715; British Pat. No. 1,409,695; Brit. J. Appl. Phys. Series 2, 2 569-576 (1969, Carley Read and Stow); Kautschuk und Gummi II WT, 138-148 (1958, de Meij); Polymer Engineering and Science, Nov. 1973, 13, No. 6, 462-468 (J. Meyer); U.S. Patent Office Defensive Publication No. T 905,001; German Offenlegungschriften Nos. 2,543,314.1, 2,543,338.9, 2,543,346.9, 2,634,931.5, 2,634,932.6, 2,634,999.5, 2,635,000.5, 2,655,543.1, 2,746,602.0, 2,755,077.2, 2,755,076.1, 2,821,799.4 and 2,903,442.2; and German Gebrauchsmuster No. 7,527,288. Reference may also be made to U.S. Patent Application Ser. Nos. 601,424 now abandoned (and the CIP thereof Ser. No. 790,977) now abandoned, 601,549 now abandoned (and the CIP thereof Ser. No. 735,958 now abandoned), 601,550 now Pat. No. 4,188,276, 601,638 now Pat. No. 4,177,376, 601,639 now abandoned, 608,660 now abandoned, 638,440 now abandoned (and the CIP thereof Ser. No. 775,882 now abandoned), 732,792 now abandoned, 750,149 now abandoned, 751,095, 798,154 now abandoned and 873,676. . The disclosure of each of these publications and applications is hereby incorporated by reference.
PTC compositions are useful, inter alia, in electrical devices comprising a PTC element in combination with another resistive element whose resistance remains relatively constant at least up to the temperature range in which the PTC element shows a very rapid increase in resistance, such other element being referred to as a constant wattage (CW) [or relatively constant wattage (RCW)] element. It is to be noted that the resistance of a CW element need only be relatively constant in the temperature range of normal operation; thus it can decrease, remain constant, or increase slowly in this range, and can exhibit PTC characteristics above normal operating temperatures of the device. Such devices are described for example in U.S. Pat. No. 4,017,715 and German Offenlegungschrift Nos. 2,543,314.1 and 2,903,442.2. In order to obtain the best results from such devices, it is necessary that the resistivities of the PTC and CW elements should be correlated throughout the temperature range of operation and in many cases that the resistivity/temperature characteristics of the elements and the contact resistance between the elements (whether bonded directly to each other, as is generally preferred, or through a layer of a conductive adhesive) should not change excessively on storage or in use, eg. due to temperature variations which take place during operation of the device. The CW compositions hitherto available are not fully satisfactory in these respects. For example, it is well known that certain conductive polymer compositions comprising an elastomer and a carbon black exhibit CW behavior, but unfortunately the resistivity of such compositions is excessively dependent on their thermal history.